Process for injection molding foam synthetic resin materials involving introduction of the expansion agent into the metering zone between injection molding steps

ABSTRACT

THE PRESENT DISCLOSURE IS DIRECTED TO A METHOD FOR THE PRODUCTION OF FOAMED MOLDED ARTICLES BY INJECTION MOLDING A HEAT-PLASTICATED MIXTURE OF A THERMOPLASTIC SYNTHETIC MATERIAL WITH A LOW-BOILING LIQUID OR GASEOUS EXPANDING AGENT IN AN INJECTION DEVICE CONTAINING A FEED ZONE, A ZONE OF INCREASING PRESSURE AND A METERING ZONE, WHEREIN A FLUID PREMIX OF A THERMOPLASTIC SYNTHETIC MATERIAL IS INTRODUCED INTO THE FEED ZONE OF THE SCREW BARREL CHAMBER OF AN INJECTION MOLDING DEVICE WHERE IT IS SUBJECTED TO THE EFFECTS OF HEAT AND PRESSURE AND SUBSEQUENTLY MIXED WITH AN EXPANDING AGENT INTRODUCED INTO THE METERING ZONE OF THE SCREW BARREL CHAMBER OF THE INJECTION MOLDING DEVICE, SAID MIXTURE BEING EJECTED UNDER HIGH PRESSURE INTO AN INJECTION MOLD WHERE THE MIXTURE EXPANDS AND FOAMS UP IN A PRESSURE-EXPANDED FASHION FORMING SAID FOAMED MOLDED ARTICLES. IN THIS METHOD THE EXPANDING AGENT IS INTRODUCED AFTER THE COMPRESSION ZONE OF INCREASING PRESSURE INTO THE METERING ZONE OF ESSENTIALLY UNIFORM-REMAINING PRESSURE OF THE SCREW BARREL CHAMBER IN SUCH A MANNER THAT THE EXPANDING AGENT IS FED INTO THE SCREW BARRED CHAMBER ONLY DURING THE TIME INTERVAL OF PLASTICATING BETWEEN TWO SUCCESSIVE INJECTION MOLDING STEPS.

Feb. 19, 1914 HANSULRICH FINKMANN ET AL PROCESS FOR INJECTION MOLDINGFOAM SYNTHETIC RESIN MATERIALS INVOLVING INTRODUCTION OF THE EXPANSIONAGENT INTO THE METERING ZONE BETWEEN INJECTION MOLDING STEPS Filed Dec.24, 1970 I 2 Sheets-Sheet l INVENTORS HANS-ULRICH FINKMAN HANS FELGERATTORNEYS 1974 HANS-ULRICH FINKMANN ETAL 3,793,416

PROCESS FOR INJECTION MOLDING FOAM SYNTHETIC RESIN MATERIALS INVOLVINGINTRODUCTION OF THE EXPANSION AGENT INTO THE METERING ZONE BETWEENINJECTION MOLDING STEPS Filed Dec. 24, 1970 2 Sheets-Sheet 2 "UnitedStates Patent US. Cl. 264-53 12 Claims ABSTRACT OF THE DISCLOSURE Thepresent disclosure is directed to a method for the production of foamedmolded articles by injection molding a heat-plasticated mixture of athermoplastic synthetic material with a low-boiling liquid or gaseousexpanding agent in an injection device containing a feed zone, a zone ofincreasing pressure and a metering zone, wherein a fluid premix of athermoplastic synthetic material is introduced into the feed zone of thescrew barrel chamber of an injection molding device where it issubjected to the effects of heat and pressure and subsequently mixedwith an expanding agent introduced into the metering zone of the screwbarrel chamber of the injection molding device, said mixture beingejected under high pressure into an injection mold where the mixtureexpands and foams up in a pressure-expanded fashion forming said foamedmolded articles. In this method the expanding agent is introduced afterthe compression zone of increasing pressure into the metering zone ofessentially uniform-remaining pressure of the screw barrel chamber insuch a manner that the expanding agent is fed into the screw barrelchamber only during the time interval of plasticating between twosuccessive injection molding steps.

BACKGROUND OF THE INVENTION The present invention relates to a processfor the pro duction of molded articles of a foamed synthetic material byinjection molding a heat-plasticated mixture of said synthetic materialwith low-boiling liquid or gaseous expanding agents wherein first afluid premix of the synthetic material is prepared by the effect of heatand pressure in the screw barrel chamber of an injection moldingmachine. This premix is thereafter mixed with an expanding agent fedinto the barrel chamber of the injection molding machine and ejectedunder a high pressure into a cooled injection mold where the mixturefoams up due to pressure-expansion and forms the foamed plastic article.

It is conventional according to German published application DAS 1,181,897 to incorporate a solid expanding agent into the raw material inthe form of granules or powder to be processed in accordance with theinjection molding method, or to add this expanding agent to the rawmaterial, and to plasticate this mixture under the influence of heat.During this procedure, the expanding agent decomposes, with gas beingseparated therefrom, but remains in solution under the effect ofexternal pressure and expands the foamable plastic only when thepressure is relieved, that is, after the mixture has entered a cavitymold. This process exhibits the disadvantage that the attainable foamdensity, when incorporating a constant amount of expanding agent intothe plastic which is employed, depends on the volume and the fiowpath(wall thickness relationship of the articles to be produced) andconsequently the amount of expanding agent must be determinedexperimentally by complicated preliminary tests 3,793,416 Patented Feb.19, 1974 for each type and size of molded article to be manufactured.Furthermore, a very nonuniform density distribution over thecross-section and over the length of the flowpath can be observed in themolded article.

Another conventional process, 'DAS l,233,578, employs the same type ofincorporation of the expanding agent. However, the expander-containingmass is first conveyed into an intermediate container in order to bepressed into the injection mold in a subsequent process step. Therefore,this process not only exhibits the disadvantages of the firstmentionedmethod, but additionally requires a technically complicatedconstruction.

Furthermore, a process for the production of molded foam articles isconventional according to DAS 1,261,660, wherein the heat-plasticatedpremix is subjected to low pressure prior to the addition of theexpanding agent. At this point, the expanding agent is added, and thetotal mixture is ejected under high pressure. This process has thedisadvantage that a special configuration of the screws is necessary,which screws are very complicated in their design. The furtherdisadvantage can be observed that the expanding agent introduced intozones of lower pressure is not immediately absorbed by the plastic melt,but, rather, is entrained in the form of individual gaseous or liquidbubbles. Additionally, there is the danger that the melt, conveyedagainst increasing pressure, does not absorb the expanding agent fedthereto, and thus the expanding agent travels backwards against theconveyance of the material and can escape through the feeding hopper. Itis impossible to obtain a uniform distribution of the expanding agentwith this procedure, and thus strongly differing pore sizes result.

SUMMARY OF THE INVENTION For the continuous extrusion ofexpander-containing thermoplastic materials, the suggestion has beenadvanced in West German patent application P 18 07 534.8 to add theexpanding agent to the plasticated premix of a thermoplastic syntheticmaterial in the screw barrel (cylinder) downstream of the zone ofincreasing pressure, in a zone of pressure which remains substantiallythe same.

It has now been found that the disadvantages of the previously knowninjection molding processes in the production of molded articles fromfoamed plastic can be avoided by adapting the process of feeding andmixing the expanding agent as employed in the continuous extrusionmethod to the injection molding method in such a manner that itcorresponds to the cyclic character of this method.

This problem is solved, in accordance with the present invention, byintroducing the expanding agent after the zone of elevated, increasingpressure into the zone of substantally uniform pressure of the screwbarrel in such a manner that the expanding agent is introduced into thescrew barrel only during the entire time period of plastication setbetween two successive injection steps.

Suitable apparatus for conducting the present process are screwinjection molding machines, as well as plungertype injection moldingmachines with screw-type preplasticating.

DESCRIPTION OF THE DRAWINGS FIG. 1 represents a longitudinal sectionthrough a schematically illustrated screw-type injection moldingmachine;

FIG. 2 is a longitudinal section through a plunger injection moldingmachine with a screw-type preplasticator; and

FIG. 3 shows pressure curves 0, b, 0 along the screw barrel forcharacteristic screw positions A, B and C of an operating cycle of thescrew-type injection molding machine. FIG. 3 also shows the pressurecurve d of a plunger injection molding machine with screw-type pre-DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process of the presentinvention for the production of thermoplastic foamed plastic articles,the premix to be foamed is fed, in granular or pulverized form, to thescrew-type injection unit (FIG. 1) via the hopper 2 of the screw 9. Thispremix can contain pore-regulating substances and additives, such aspigments, fillers, flame retardants, and antistats. The screw'9 isaxially displaceable in the screw barrel 8 and rotates about its ownaxis. The screw 9 is driven by the drive mechanism 1. The screw 9 has alength L of 15-25 times the outer diameter of the screw and exhibitssuccessively arranged zones of differing geometry. Zone E represents thefeed zone, with a length which is '6-12 times the outer diameter of thescrew. Zone K represents the compression zone, also called theconversion zone, and has a length of 0.5-6 times the outer diameter ofthe screw. Zone M represents the metering zone of the screw 9 and has alength which is 6l0 times the outer diameter of the screw. The screw tip11 is provided with a backflow blocking means 10 preventing theplasticated mixture from flowing back during the injection thereofthrough a shut-off nozzle 12 via a sprue 13 into the mold cavity 14 ofthe injection mold 15. The screw barrel 8 can be heated by means ofheating elements 3.

At the beginning of the plasticating process in the screwtype injectionmolding machine (FIG. 3, A), the pressure in the screw barrel builds upfrom the hopper 2 of the machine toward the tip of the nozzle andexhibits the course of curve a in FIG. 3. Shortly before termination ofthe plasticating step (FIG. 3, B), the pressure assumes the curve b.

In the zone of the essentially uniform or increasing pressure, the point7 (FIG. 3, A and B; FIG. 1) is selected ior tllie feed site of theexpanding agent into the screw 1 arre The expanding agent is introducedin metered quantities from a storage tank 4 (FIG. 1) with the aid of ametering pump at the inlet nozzle 7, through a back pressure valve 6,into the heat-plasticated premix, the rotating screw 9 moving in thedirection toward the drive mechanism 1, due to the force of theaccumulating mass in front of its tip.

During this process, the barrel 8 is sealed by means of a shut-offnozzle 12 so that no plasticated melt can escape from the barrel at thebarrel end. Once a sufiicient amount of melt has been prepared for theproduction of the article, the screw is arrested by shutting off thescrew drive motor in the drive mechanism 1. At the same time, the valve6 is closed, the pump 5 is cut off, and the metered feeding of theexpanding agent for the instantaneous operating cycle is terminated.Thereafter, the screw is shifted axially in the direction of the moldingtool 15. The backflow blocking ring 10, which is under a high pressure,seals the melt with respect to the screw, so that no melt can flowacross the flights of the screw in the direction toward the feedinghopper 2. The shut-0E nozzle 12-is opened, and the melt passes underpressure through the sprue 13 into the mold cavity 14 of the injectionmold 15 FIG. 3, C, shows this position of the screw, and the curve 0shows the associated pressure after opening the shut-off nozzle 12during the injection step into the mold cavity 14 (FIG. 1). Curves a, band 0 show that the metered feeding point 7 is not under any higherpressure, during the entire cycle, than at the beginning of the meteredfeed.

In case a plunger-type injection molding machine. is employed, with ascrew-type preplasticator (FIG. 2), the process of the present inventiontakes place as follows:

The screw 25 in the cylinder (barrel) 24 rotates only about its own axisand is not axially displaceable. A

plunger cylinder 21 with a movable plunger 20 is associated with thescrew barrel 24. Both cylinders are heated with the aid of heatingelements 18.

In FIG. 3, position D, the screw barrel of this machine is illustrated,wherein the length of the path from the inlet of the hopper 16 to thetip 26 of the screw (see FIG. 2) is drawn in adaptation (to the samescale) to the length of the'corresponding path of the screw barrel ofthe screw-type injection molding machine. Curve d shows the course ofthe pressure along the thus-represented path during the plasticatingstep.

This illustration is to clarify that, when using the preplasticizingscrew, the point of feeding the expanding agent 22 in the screw barrelzone of the essentially uniform or increasing pressure is selectedentirely analogously to the procedure followed in connection with thescrewtype injection molding machine of FIG. 1.

' The expanding agent is fed in metered quantities from the storage tank17 with the aid of the metering pump 19 at the feed point 22, via acheck valve 23, into the heat-plasticated premix. The preplasticatedmass provided with the expanding agent passes from the screw barrel 24via a duct 27 heated with the heating elements 18 into the plungercylinder 21 and is from there forcibly conveyed through a shut-off noule29 by way of the sprue 30 into the mold cavity 31 of the molding tool32, with the aid of the plunger 20. In order to prevent the blackfiow ofplasticated melt during the injection step from the cylinder 21 via theduct 27 into the screw barrel 24, the cylinder 21 is closed off withrespect to the duct 27 by means of a check valve (back pressure valve)28.

This valve 28 is controlled in dependence on the cycle in such a mannerthat it is closed at the beginning of the plunger movement in thedirection toward the injection nozzle 29 and is again opened upontermination of the injection step.

Simultaneously with the closing of the valve 28, the metering pump 19for the expanding agent is arrested, and the valve 23 is closed. Theactuation of the metering pump 19 and the opening of the valve 23 occurdirectly after the plunger sealing valve 28 has been opened.

In both types of injection molding machines, the necessary temperatureprogram is dependent on the particular synthetic material employed, andis determined by preliminary experimentation.

Synthetic polymeric materials which can be used in the present inventioninclude those having a relatively low viscosity for the ambient melttemperature at the metering point of the expanding agent and thoseexhibiting a softening point range which is not too broad. Suitablematerial include thermoplastic synthetic materials, such as polyolefins,e.g. polyethylene, polypropylene, etc., polyvinyl compounds, e.g.polystyrene, polyvinyl chloride etc., polyamides, e.g. nylon, and thelike.

Suitable pore-controlling agents which can be utilized includecarbonates and bicarbonates of sodium in mixtures with an organic acid,such as citric acid and optionally with inorganic compounds.

Expanding agents which can be employed in the present process compriselow-boiling, liquid substances, preferably aliphatic hydrocarbonsboiling between 20 and 70 C., such as pentane, hexane, etc., orchlorinated hydrocarbons, such as trifiuorotrichloroethane,tetrafluorodichloroethane, and monofluorotrichloromethane. In additionto these low-boiling, liquid hydrocarbons, it is also possible to employgaseous expanding agents, such as propane, butane, etc., by itself or asmixtures, or blends with liquid expanding agents.

The amount of expanding agent to be utilized depends on the size of thearticle, the thermoplastic synthetic material to be employed, and thefoam material density to be attained, as well as on the expanding agentemployed in that particular case, and is about 0.1 to 10% by weight,based on the total mixture.

The process of the present invention permits the production of foamedmolded articles by the defined metered addition of the expanding agentin dependence on the desired density of the molded article to beproduced. In this connection, the process avoids the disadvantagesoccurring in a continuous introduction of the expanding agent togetherwith the plastic and in an introduction of the expanding agent under lowpressure into the heat-plasticated premix, which produces. theoccurrence of nonuniform densities and the backfiow of the expandingagent. As compared to the injection molding process employing anintermediate container, the process of the present invention has theessential advantage of a simpler technical operation.

The following examples are merely exemplary of the present invention andare not to be considered as limiting the scope of the present invention.

EXAMPLE 1 A screw-type injection molding machine is employed having ascrew of a diameter of 45 mm. The screw is subdivided into three zonesand has a length of 209:, wherein x represents the screw diameter. Thescrew has a feed zone length of x, a compression zone length of 2x, anda metering zone with a length 8x. The screw flight depth of the feedzone is 5.8 mm. and the flight depth of the metering zone is 1.5 mm.,resulting in a compression ratio of 3.87: 1. The tip of the screw isprovided with a backfiow blocking means, and the screw barrel isequipped at the tip with a spring-loaded shut-off nozzle. The pitch ofthe screw is 1x. The point where the expanding agent (in this case,monofluorotrichloromethane) is introduced through the nozzle is at adistance of 2x from the beginning of the metering zone, calculatedtoward the tip of the screw, when the screw is in the frontmost positionin the screw barrel.

Polystyrene was processed having a bulk density of 1.05 g. cm. accordingto DIN (German Industrial Standard) 53 479 and a viscosity of 90,measured along the lines of DIN 53 726 (viscosity number) as well as aK-value of 60, measured at 20 C. according to the polyvinyl chloridestandard. At a metering path of 117 mm., corresponding to 2.6x, 170 cc.of melt, the material was processed within 20 seconds, at a barreltemperature adjustment from the feeding hopper to the nozzle whichincreased from 180 C. to 200 C. to 210 C., and a nozzle temperature of210 C., with a mass temperature of 226 C. at a screw ram pressure of 60kp./cm. During this preparation of the melt, 2.5 parts by weight ofmonofluorotrichloromethane was introduced at the above-described feedpoint into the melt under a pressure of 82 kp./cm. The screw speed ofrotation was 60 rpm. during this plasticating process. Thereafter, underan injection pressure of 800 kp./cm. the polystyrene melt, containingthe expanding agent in a finely distributed form, was pressed (forced),after the opening of the shut-ofl? nozzle, via a central sprue of 2.5mm. diameter, into the cavity of a closed and cooled mold. The injectiontime was 1.2 seconds. The melt foamed only after being deposited intothe mold cavity and filled this cavity completely in the foamedcondition. After a cooling period of 60 seconds, the article could beremoved from the mold. During the cooling-01f period of theinjection-molded article, the expander-containing melt, which had notyet been expanded, was already prepared in the plasticating unit for thesubsequent injection step. The dimensions of the article included adiameter of 250 mm. and a wall thickness of 6.9 mm. The surface of thearticle was not foamed up to a depth of 0.3 mm., whereas a very uniformpore structure was observed in the interior of the article, with anaverage pore diameter of 0.15 mm. The average density of the moldedarticle was 0.5 g./cm.

EXAMPLE 2 In a plunger-type injection molding machine with screwpre-plasticater, a lil-less box having the dimensions of 250 x 250 mm.with a height of 110 mm. and a wall thickness of 10 mm. is produced viaa central rod-shaped sprue having a length of 30 mm. and flaring from 4to 6 mm.

This box has an average density of 0.63 g./cm. and is produced ofexpandable impact-resistant polystyrene (as specified in Example 1). Inthe screw barrel, a screw is arranged having a screw diameter of 45 mm.,with a length of 15x, with x being the screw diameter. The lengths ofthe feed and compression zones of the screw amount, in each case, to 3x,whereas the metering zone has a length of 8x, at a constant flight pitchof 1x. The depth of the screw flights in the feed zone is 6 mm., and theflight depth in the metering zone is 1.7 mm., resulting in a flightdepth ratio of 3.53:1. The screw cannot be displaced axially. It isdriven by the screw drive motor and rotates only about its own axisduring the plasticating step. T hepoint where themonofluorotrichloromethane is introduced through the nozzle as theliquid expanding agent is 1.5 x after the start of the metering zonetoward the tip of the screw. The screw, which rotates about its axis atrpm, plasticates the granulated polystyrene fed via the feeding hopperand forces the same, during this plasticating step, continuously via aheated feed duct provided with a check valve under a pressure of kp./cm. into a likewise heated plunger cylinder having an internal diameterof 65 mm.; the plunger moves backwards during this procedure, due to theconveying pressure, against a back pressure which is 1-2 kp./cm. lowerthan the conveying pressure of the screw. The expandercontaining melt,which is not yet expanded, is maintained in the plunger cylinder, whichis closed oil by a shut-0E nozzle, under this pressure. For producingthe above-mentioned article, the screw plasticates and conveys, at 120rpm. over a period of 42 seconds, about 1,000 cc. of expander-containingmelt into the plunger cylinder. The plunger moves backward at the sametime over a stroke of 300 mm. During this plasticating and conveyingtime, an amount of expanding agent of 2.8% by weight, based on theplasticated melt, is introduced with the aid of the expanding agentmetering pump, at a pressure of kp./ cm. at the above-described point ofthe screw barrel, into the melt. Once the expander-containing amount ofmelt has passed into the plunger cylinder, the plasticating process issimultaneously arrested by discontinuing the screw rotation, the checkvalves at the feed point for the expanding agent and in the feed duct tothe plunger cylinder are closed, and the shut-off nozzle is opened. Themovement of the plunger in the direction toward the nozzle introducesthe melt, under a pressure of 850 kp./cm. and within a period of 1.8seconds through the nozzle by way of the sprue into the cooled andclamped box-shaped mold, where the melt is expanded under the effect ofthe expanding agent pressure, thus filling the entire mold cavity. Anunfoamed surface of 0.6 mm. and a core built up from closed cells withan average cell diameter of 0.2 mm. are thus formed. The expanded meltsolidifies, and after 85 seconds, the above-described article is removedfrom the mold. The total density of the article is 0.63 g. cm. The totalcycle time is 90 seconds. Immediately after forcing the melt into themold cavity, the shut-01f nozzle is simultaneously closed, the meteringpump for the expanding agent is actuated, and the screw, at theabovementioned screw speed, commences with the preparation of the meltfor the subsequent injection molding step. Due to the metering pressureof the expanding agent, the check valve at the feed point for theexpanding agent is opened and the expanding agent is forced into themelt. Furthermore, the check valve at the feed duct to the plungercylinder is opened due to the screw conveying pressure of the melt.Additionally, the plunger back pressure is reduced from 850 kp./cm. to128 kp./cm. so that the melt fed by the screw can again pass into theplunger cylinder, pressing back the plunger proper. Once a sufficientamount of melt has been prepared, the latter remains in the plungercylinder under heat until the mold, after removal of the articleinjection-molded in the previous cycle, is closed again so that the newinjection molding step, as described above, can begin. The temperaturesin the three heating zones of the screw barrel from the feeding hopperto the tip of the screw are controlled to be 210 C., 210 C., and 210 C.,respectively. The feed duct from the screw barrel to the plungercylinder is provided with two heating zones maintained at a constanttemperature of 210 C., and the heating elements of the plunger cylinderare regulated to be at a constant temperature of 250 C. With thesetemperature values, the melt temperature, measured upon exit from thenozzle of the plunger cylinder, was 223 C.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such varia-. tions are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be apparent to one skilled in the art areintended to be included.

What is claimed is:

1. In a method for the production of foamed molded polymeric articles byinjection molding a heat plasticated mixture of a thermoplasticsynthetic molymeric material suitable for injection molding with alow-boiling organic liquid or gaseous expanding agent or mixturesthereof in an injection device wherein a fluid premix of saidthermoplastic synthetic polymeric material is prepared by the eifects ofheat and pressure in a screw barrel chamber of an injection moldingdevice and the fluid premix is subsequently mixed with said expandingagent and the resultant mixture is ejected under pressure by a screw insaid chamber into an injection mold where the mixture expands and foamsup in a pressure-expanded fashion to form said foamed molded polymericarticles, the improvement which comprises effecting preparation of saidpremix in an injection molding device having a feed zone, a compressionzone of increasing pressure, and a metering zone of essentially uniformremaining pressure and introducing said expanding agent after thecompression zone into the metering zone of the screw barrel chamber onlyduring the time interval of plasticating between two successiveinjection molding steps.

2. The method of claim 1, wherein the expanding agent is introducedbetween two successive injection molding steps in a uniform amount perunit of time.

3. The method of claim 1, wherein the injection of the mixture ofthermoplastic synthetic polymeric .material and the expanding agent iseffected by the axial movement of a screw in the screw barrel chamber.

4. The method of claim 3, wherein the screw is not rotating during theinjection.

5. The method of claim 1, wherein the mixture of thermoplastic syntheticpolymeric material and the expanding agent is introduced into thechamber of a plunger,- type injection molding device from where themixture is injected into the injection mold where the mixture expandsand foams up forming said foamed molded polymeric articles.

6. The method of claim 5, wherein the screw is not rotating during theinjection of the mixture into the in jection mold.

7. The method of claim 1, wherein the expanding agent is introduced at adistance which is greater than the pitch of the screw from the beginningof the metering zone, calculated toward the tip of'the screw, when thescrew is in the frontmost position in the screw barrel, that saidexpanding agent does not enter said compression zone.

8. The method of claim 7, wherein the distance of the introduction ofthe expanding agent is 1.5 times the pitch of the screw.

9. The method of claim 7, wherein the distance of the introduction ofthe expanding agent is twice the pitch of the screw. I

10. The method of claim 7, wherein the screw has a length of 15 to 25times the outer diameter of the screw, the feed zone has a length of 6to 12 times the outer diameter of the screw, the compression zonehas alength of 0.5 to 6 times the outer diameter of the screw and themetering zone has a length of 6 to 10 times the outer diameter of thescrew.

11. The method of claim 7, wherein the screw has a length 20 times theouter diameter of the screw, the feed zone has a length of 10 times theouter diameter of the screw, the compression zone has a length of 2times the outer diameter of the screw and the metering zone has a lengthof 8 times the outer diameter of the screw; wherein the screw flightdepth of the feed zone is 5.8 mm. and the flight depth of the meteringzone is 1.5 mm. resulting in a compression ratio of 2.81 to 1.

12. In a method for the production of foamed molded polymeric articlesby injection molding a heat-plasticated mixture of a thermoplasticsynthetic polymeric material suitable for injection molding with alow-boiling organic liquid or gaseous expandng agent or mixtures thereofin an injection device wherein a fluid premix of said thermoplasticsynthetic polymeric material is prepared and plasticated in a screwbarrel chamber of an injection molding device and the plasticated fluidpremix is subsequently mixed with said expanding agent and the resultingmixture is ejected under pressure by a screw in said chamber into aninjection mold where the mixture expands and foams up in apressure-expanded fashion to form said foamed molded polymeric articles,the improvement which comprises effecting preparation and plasticationof the premix in an injection molding device having sequentially a feedzone, a compression zone of increasing pressure, and a metering zone ofessentially uniform remaining pressure and introducing said expandingagent into the metering zone at'a distance from the end of thecompression zone which is at least equal to the pitch of the screw ofthe metering zone, mixing the expanding agent with the heat-plasticatedpremix, wherein said expanding agent is fed into the screw barrelchamber only during the time interval of plasticating between twosuccessive injection molding steps.

References Cited UNITED STATES PATENTS 3,133,316 5/1964 Arpajian 264-329UX 3,140,332 7/1964 Brown 264-329 3,191,233 6/1965 Linderoth 264-329 X3,396,431 8/ 1968 Kovach et a1 264-329 X 3,436,446 4/1969 Angell, Jr.264-51 3,551,947 1/1971 Jennings 264-40 UX 3,647,309 3/1972 Thompson425-162 X 3,658,973 4/ 1972 Aykanian 264-53 3,268,636 8/1968 Angell, Jr.264-51 3,440,309 4/ 1969 Breukink et al 264-53 3,285,865 11/1966 DelBene 264-51 3,145,240 8/1964 Proulx et al. 264-53 3,001,956 9/1961Meinel 264-51 3,1627703 12/1964 Eyles 264-51 3,697,204 10/ 1972 Kyritsiset a1. 264-Dig. 83

FOREIGN PATENTS 1,142,230 1/ 1963 Germany 264-329 OTHER REFERENCESWhittington, Lloyd R., Whittingtons Dictionary of Plastics, Stamford,Conn. Technomic, 1968, pp. 18, 179, 180, 181, 221, 251.

Brydson, I. A., Plastics Materials, Princeton, N.I., D. Van Nostrand,1966, pp. 209-212.

Wire and Cable Coaters Handbook, 1st edition, Wilmington, Del., E. I. duPont de Nemours and Co., Inc., 1968, pp. 18-21; 43-45.

PHILIP E. ANDERSON, Primary Examiner U.S. Cl. X.R. 264-328, 329, 349,Dig. 5, Dig. 83;.425-249, 817

